Communication systems are used to transfer information, for example between a source and a destination, via a communication link, or transmission channels. There are many different types of communication links, for example, wired (e.g., conductors, fiber optic cables) or wireless (e.g., microwave links, satellite links,), and combinations thereof, each of which may be public or private, dedicated, and/or shared, e.g., a network.
The are a variety of types of information that may be sought to be transferred, for example, sounds, images, and combinations thereof just to name a few.
Information to be transferred by the communication system may be converted, prior to transmission, to a form that is adapted to transmission over a transmission channel. For example, sounds and images may be converted into signals, e.g., electrical, electromagnetic, and/or optical signals, depending on the characteristics of the transmission channel.
Converting information into a signal that is to be transmitted may involve use of a standard or a protocol. For example, a signal that is to be transmitted by way of a phone line may be generated according to a plain old telephone system (POTS) standard. Alternatively, such a signal may be generated according to other standards including but not limited to emerging digital subscriber line (xDSL) standards including DSL, HDSL, HDSL2, VDSL, asymmetrical digital subscriber line (ADSL), and symmetrical ADSL. Yet another standard is integrated services digital network (ISDN).
Standards or protocols may also be used for television signals, which for example, may be broadcast via radio waves or transmitted via a coaxial cable (cable television). A television signal may, for example, be generated according to a standard definition television (SDTV) standard or a high definition television (HDTV) standard. In addition, a television signal may comprise a video component and an audio component.
Although not a requirement, each standard may make use of, or occupy, a particular signal frequency band or spectrum. For example, typical voiceband activity on phone lines in accordance with POTS standards typically occupy the 0 to 4 kiloHertz (KHz) frequency band. Present day analog modems (V.34, V.90 etc) also use the 0 to 4 KHz frequency band. FIG. 1 shows a representation of a signal frequency spectrum of POTS 40. Higher capacity digital communications standards may use greater bandwidths than that of POTS. Examples of such higher capacity digital communication standards include but are not limited to ISDN (integrated services digital network), which uses 0 to 80 kHz, and emerging digital subscriber line (xDSL) standards. Present variations of DSL include DSL (160 kbits/sec, full-duplex), and HDSL (900 kbits/sec, full-duplex). In development are the HDSL2 (1.8 Mbits/sec, full-duplex) and VDSL (13–52 Mbits/sec, full duplex) standards. In asymmetrical digital subscriber line (ADSL) systems, the upstream rate to the central office (CO) is different than the downstream rate to the subscriber line, e.g., 384 kbits/sec to 640 kbits/sec, and 1.5 Mbits/sec to 8 Mbits/sec, respectively. Television standards may use other frequency bands.
A single communication link may support multiple communication standards. For example, a phone line may carry ADSL and POTS or ISDN and ADSL. FIG. 2 shows a representation of a signal frequency spectrum of POTS 50 and signal frequency spectrum of ADSL 52.
FIG. 3 illustrates a system 90 that supports POTS and ADSL. The system includes a phone line 92 that carries a signal (tip/ring) having a POTS component and an ADSL component. The phone line 92 is connected to an analog splitter network 94. The analog splitter network 94 has a low pass filter section 96 that is coupled to a POTS signal path 98, and a high pass filter section 100 that is coupled to an ADSL signal path 102. In this instance, the low pass filter section 96 filters out the ADSL component so that the POTS signal path 98 receives only the POTS component. The high pass filter section 100 filters out the POTS component so that the ADSL signal path 102 receives the ADSL component.
The POTS signal path 98 includes an analog section 104, a converter section 106 and a digital filter section 108. The analog section 104 of the POTS signal path 98 includes an isolation barrier 110 (e.g., an isolation transformer) that ground isolates the receiver/transmitter from the phone line 92. The isolation barrier 110 has a differential connection to a POTS analog front-end (AFE) 112. After the POTS AFE 112, the POTS signal path 98 splits into a receive side and a transmit side. As to the receive side, the POTS AFE 112 outputs an analog signal this is supplied to an analog to digital converter (ADC) 114. The ADC 114 generates a sampled data stream, which is supplied to a digital filter 116, e.g., a decimation filter. The digital filter 116 outputs a sampled data stream having a lower sample rate than that out of the ADC, for example, 7 to 48 kHz. Each sample of the sampled data stream typically comprises 16 bit digital data. The sampled datastream is input to a DSP 118 which may be part of a host computer and which may process the data in executing voice (e.g. speaker phone) or modem (e.g. V.90, V.34, Fax etc) applications. Another interface such as PCI 120 may be used to pass data between the DSP 118 and a computer (not shown).
As to the transmit side of the POTS signal path 98, the DSP 118 provides a sampled data stream to a digital filter 122, e.g., an interpolation filter. As with the data out of the decimation filter 116, the sampled data stream into the interpolation filter 122 typically has a sample rate of 7 to 48 kHz. Each sample of the sampled data stream typically comprises 16 bit digital data. The interpolation filter 122 outputs a data stream that has higher sample rate than that out of the DSP 118. The data stream is supplied to a DAC 124. The DAC 124 converts the sampled data stream to an analog signal that is supplied to the POTS AFE 104, whereby the analog signal is coupled to the isolation barrier 110.
The ADSL signal path 102 is similar to, but separate from, the POTS signal path 98 and includes an analog section 126, a converter section 128, a digital filter section 130, which is coupled to a DSP 132. The parameters of the ADSL signal path 102 are selected in view of the ADSL spectrum. The sample rates of the DAC 124 of the ADSL signal path are typically 552 kHz, or 1.104 MHz. The sample rates of the ADC 114 of the ADSL signal path are typically 1.104 MHz or 2.208 MHz. Although shown as each having a DSP, the POTS signal path 98 and the ADSL signal path 102 may instead be fed to a single DSP.
The digital interpolation and decimation filters typically have low-pass characteristics. For example, the POTS interpolation and decimation filters 122, 116 may start to roll-off at 4 kHz. The ADSL interpolation filter 136 rolls-off at 138 kHz. The ADSL decimation filter 138 rolls-off at either 552 kHz or 1.104 MHz.
FIG. 4 illustrates a portion 150 of a communication system that supports television. The communication system includes a transmitter 152 and a receiver 154. The transmitter 152 has an analog video signal that occupies a frequency band from 0 to 4.2 mega Hertz (MHz), and an analog audio signal that occupies a frequency band from 0 to 20 kHz. The analog audio signal is supplied to an FM modulator 156 that outputs an FM modulated audio signal having a carrier frequency of 4.5 MHz. FIG. 5 is a representation of a signal frequency spectrum of the analog video signal. FIG. 6 is signal frequency spectrum of the combined RF modulated audio and video signal. The FM modulated audio signal and the analog video signal are supplied to a summer 158. A sum is supplied to an RF modulator 160, which outputs an RF modulated signal having a carrier frequency in one or more UHF and/or VHF frequency bands in accordance with the standards defined by the Federal Communication Commission for terrestrial television broadcast. The RF modulated signal, which has a video component and an audio component, is then transmitted 162.
At the receiver 154, a signal is received by an antenna 164 and supplied to an RF demodulator 166 that demodulates the signal down to a baseband signal that is a sum of the video signal component and the FM modulated audio signal component. The baseband signal is supplied to an analog splitter network 168 which has a band pass filter section 170 that is coupled to an audio signal path 172, and a low pass filter section 174 that is coupled to a video signal path 176. The band pass filter section 170 filters out the video component so that the audio signal path 172 receives only the audio component. The low pass filter section 174 filters out the audio component so that the video signal path 176 receives the video component.
The audio signal path 172 includes an FM demodulator 178 that receives the FM modulated audio signal and outputs a demodulated audio signal, which is supplied to an ADC 180. The ADC 180 generates a sampled data stream, which is supplied to a digital filter 182, that outputs a sampled data stream, which is input to a DSP 184 e.g. a host computer in a television set (not shown). The video signal path 176 includes an ADC 186, and a digital filter 188 coupled to a DSP 190. The ADC, digital filter and DSP functions are sometimes referred to as a video decoder.
FIG. 7 illustrates an analog front end (AFE) 192 for a communication system that uses a phone line. The analog front end 192 has a transmit side that includes a gain stage, which receives the analog signal from a DAC e.g. DAC 139B of the ADSL signal path 102. The gain stage outputs an amplified signal, which is supplied via a signal line to a termination impedance (Zt) 196. The termination impedance 196 is coupled to the splitter/phone line, which carries the signal to be transmitted. The electrical connections may be single ended, as shown in FIG. 7, or differential.
The termination impedance 196 helps provide line matching with the phone line to help eliminate signal path reflections that can degrade the signal on the phone line. The impedance of the phone line is frequency dependant and can vary from several kilo-Ohms at DC to 100 Ohms in the MegaHertz range. Signal protocols that occupy different frequency bands may warrant different termination impedances. A resistor (not shown) is commonly used as the termination impedance 196. For POTS, a fixed 600 Ohm value is used. For ADSL, a fixed 100 Ohms value is used. For ISDN, a fixed 120 Ohm value is used.
The analog front end 192 further includes a receive side that includes an echo cancel circuit 198, which receives the amplified signal from the gain stage 194. The output of the echo cancel circuit 198 is subtracted 197 from the phone line signal to remove most of the local transmit (Tx) signal, and the result is supplied to ADC, e.g. ADC 139A, of the ADSL signal path 102 (FIG. 3). The echo cancel circuit 198 and the subtraction 197 are sometimes combined into a hybrid network 199. This results in the ADC seeing only the receive (Rx) portion of the phone line signal. The amount of residual local Tx signal on the ADC input is dependent at least in part on how well the termination impedance 196 matches the phone line impedance.
Communication systems may require that the receiver be phase and/or frequency locked to the transmitter. In such instances, a phase locked loop (PLL) is typically used to help maintain such phase and/or frequency lock.
FIG. 8 illustrates one type of PLL. In this illustration, a voltage controlled oscillator 201B generates an output signal on line 201C. The output signal is phase/frequency locked to a reference signal on signal line 201D, as follows. A voltage controlled oscillator generates an output signal. The output signal is supplied to a phase detector 201E, which also receives the reference signal on signal line 201D. The phase detector generates a correction signal on line 201F in response to a difference in frequency/phase the between the two signals. The correction signal is filtered 201G and supplied to the voltage controlled oscillator 201B, which in response, generates the output signal.
FIG. 9 is a block diagram of a portion of the ADSL signal path of the communication system of FIG. 3, showing a prior art phase locked loop 200. In ADSL applications a single pilot tone frequency is included in the transmitted signal for the PLL to use as a reference. The PLL 200 shown in FIG. 9 includes a voltage controlled crystal oscillator (VCxO) 204 that produces a variable frequency clock, which is supplied to the ADC 139A of the ADSL signal path 102 (FIG. 3). The ADC 139A responds by producing a data stream having a variable sample rate, which is passed through the ADSL decimation filter 138 (assuming that the receiver 90 employs oversampling), which further receives the variable frequency clock from the VCxO 204 and applies fixed ratio decimation to generate a received data stream also having a variable sample rate, but divided down compared to that from the ADC 139A. The real and imaginary components of the pilot tone are extracted by an FFT function 206. The extracted components are input to a phase detector 208 that produces an up/down signal, which is passed through a digital filter 210 and DAC 212 (typically 8 to 10 bits) to generate an analog signal used to control the VCxO 204, and thereby maintain phase and/or frequency lock between the transmitter and the receiver 90. The control range of the PLL 200 is typically +/−100 parts per million (ppm). The variable frequency clock from the VCxO 204 is typically further supplied to the ADSL interpolation filter 136 (assuming that the receiver 90 employs oversampling) and the DAC 139B in the transmit side of the ADSL signal path 102.